This invention relates to diagnostic apparatus for a fuel composition sensor of the type which is responsive to the dielectric constant of a mixture of two fuels to generate a fuel composition signal indicating the relative concentration of the two fuels. Such a sensor may be, for example, a capacitive sensor in which the fuel mixture to be sensed forms the dielectric between capacitor plates. If one of the fuels has a significant variation of dielectric constant with temperature, the output signal of such a sensor may vary with fuel temperature at high concentrations of the one fuel. In a fuel mixture of gasoline and methanol, for example, the output of such a sensor becomes significantly more dependent on fuel temperature as the concentration of methanol increases. Therefore, such a sensor may be provided with an accompanying fuel temperature sensor in the same package so that the fuel composition sensor output may be corrected for fuel temperature as required.
The diagnostic apparatus of the invention is of the type which detects failures such as a disruption of communication between the sensor(s) and an associated control responsive thereto by monitoring the fuel temperature sensor and fuel composition sensor outputs for values within predetermined allowed ranges. However, the diagnostic apparatus is desirably also used to detect contaminated fuel. Although a sensor will usually fail by shorting to one of the power supply terminals, in which case constant limit references may be acceptable, contaminated fuel may not produce such a pronounced failure. Rather, the contaminated fuel may produce an output voltage which, although outside the expected voltage range at a particular fuel temperature, is not at or near either of the voltage supply voltages. Therefore, to the extent that the fuel composition sensor output is temperature dependent, the use of a fixed allowed fuel composition range might provide an erroneous result where the temperature dependence is greatest: namely, for high concentrations of the one fuel, such as methanol, which exhibits a strong variation of dielectric constant with temperature.